The present embodiments relate to wireless communication systems and are more particularly directed to a wireless frequency hopping system with a receiver having a filtered adaptive slicer.
Wireless networks are becoming increasingly popular, and there has been improvement in many aspects of such networks. Various improvements relate to the use of a wireless network for a variety of devices that are typically within fairly close distances of one another, such as in the range of 10 meters or less. In the current state of the art, such a network is sometimes referred to as a personal area network (“PAN”) and it may include, by way of example, a keyboard and a printer, each of which communicates in a wireless manner with a mutual computer that is also part of the PAN. Other wireless devices (e.g., personal organizers, cell phones, and still others) also may be implemented to communicate at either the PAN level or at much greater distances. In any event, the term network is used in this document to describe a system consisting of an organized group of any of various types of intercommunicating devices.
Devices within a wireless network may communicate using one of various different protocols or the like, where one currently popular approach is known in the art as spread spectrum frequency hopping and is sometimes referred to more simply as frequency hopping. In frequency hopping, a network transmitter transmits different packets of information at different frequencies such as in an effort to reduce the chance that the packets will interfere or “collide” with packets transmitted at different frequencies by a transmitter in a different network. The change between frequencies, that is, from one frequency to another, is said to be a “hop” between the frequencies. Thus, the transmitter has a corresponding frequency hopping sequence which specifies the various different frequency bands along which the transmissions are sent. The receiver likewise is informed of and operates in response to the frequency hopping sequence so as to properly receive and demodulate the transmissions. The goal of such an approach is that each packet from a first network is transmitted at a frequency which neither overlaps nor is near enough to a frequency at which a second network is transmitting. Further in this regard, some systems transmit each successive single packet, commonly referred to as a time slot and having a duration of 625 microseconds, at a different frequency; thus, the transmitter is “hopping” to a different frequency for each packet, where the so-called Bluetooth protocol is an example of such a system. Bluetooth is a fairly new standard for radio transmissions in the 2.4 GHz ISM (Industrial, Scientific, and Medical) band, and it uses frequency hopping across a certain number of carrier frequencies, where the number of total carrier frequencies is presently set by standards which differ in various geographies. Alternatively, others systems (e.g., IEEE 802.11) transmit a first set of multiple packets at a first frequency, and then hop to a second frequency to transmit a second set of multiple packets, and so forth for numerous different sets of packets at numerous different respective frequencies.
While frequency hopping has proven itself as a beneficial protocol in wireless networks, it also has certain limitations and drawbacks due to frequency variations in both the transmission and receipt of a packet. For example, for a packet communicated in a Bluetooth communication at a carrier frequency of 2.4 GHz, each bit in that packet represents either one of two binary values based on an additional change in a modulation frequency equal to ± a value which is commonly 160 kHz, but based on implementation this value of 160 kHz could be in a range of 140 to 175 kHz. For the sake of a consistent example, in this document the change in modulation frequency is assumed to be equal to 160 kHz. Thus, for a first binary value (e.g., 1), the bit is ideally modulated at a carrier frequency of 2.4 GHz+160 kHz, whereas for a second binary value (e.g., 0), the bit is ideally modulated at a frequency of 2.4 GHz−160 kHz. This change in frequency of ±160 kHz is sometimes referred to in the art as a frequency offset In any event, error in the transmitting station's clock can cause a variation up to ±75 kHz, and error in the receiver station's clock can cause a variation up to ±50 kHz. Thus, there is the potential of a total of ±125 kHz (i.e., ±75 kHz ±50 kHz) in frequency variation in a communication between the transmitter and receiver, which therefore is a considerably large value relative to the frequency offset of 160 kHz. Clearly, therefore, a technique must exist to reduce the effect of the frequency variation so as to properly demodulate the actual data encoded by the frequency offset of 160 kHz.
In a Bluetooth system, techniques are implemented to address the frequency variations described above and they typically are performed at the same time as synchronization. Specifically, after an initial synchronization between the Bluetooth master and slave, each receiver is configured to re-synchronize itself to a transmitter's clock at the beginning of receipt of each packet from that transmitter. More particularly, the transmitter inserts a channel access code (“CAC”) at the beginning of each packet so that the receiver can use the CAC to re-synchronize itself to the transmitter's clock. The CAC consists of a preamble pattern (e.g., 1010 or 0101) followed by a 64-bit synchronization word. The receiver includes circuitry to thereby detect the CAC, which itself also introduces a ±10 bit uncertainty as to the exact location or occurrence of the CAC. As the CAC is being properly detected, the receiver also includes circuitry, as further detailed later, to correct for the frequency variations in both the transmission and receipt of the packet. Thus, once the receiver detects the CAC and also determines the frequency variation, the receiver is considered synchronized to the transmitter and can decipher the remaining data in the packet following the CAC by determining the frequency offset for each bit (i.e., either +160 KHz or −160 kHz), where this deciphering operation is assisted by the frequency variation determination made while the CAC was processed.
While the preceding approach has provided satisfactory synchronization results in frequency hopping wireless communication systems, it is observed in connection with the present inventive embodiments that such an approach also may be improved. Specifically and as also detailed later, the prior art includes a receiver for performing the synchronization and frequency variation detection functions, where the latter function is performed in part by an adaptive slicer circuit. The adaptive slicer circuit endeavors to reduce the effect of frequency variation in the received signal by determining a direct current (“DC”) voltage offset adjustment to be made to the DC voltage which is derived from the received frequency modulated signal. However, the present inventors have observed that this approach is susceptible to error due to the presence of noise in the processed signal. Indeed, such noise results in an estimated 0.8 dB loss for the performance of the receiver's adaptive slicer as compared to an ideal slicer which would know the input DC offset exactly. Accordingly, there exists a need to improve upon this performance drawback, and this need is addressed by the preferred embodiments described below.